Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials, i.e. liquids and solids, which constitute the so-called particulates or particulate matter. Emissions treatment systems for diesel engines must treat all of the components of the exhaust to meet the emission standards set by the various regulatory agencies throughout the world.
The total particulate matter emissions of diesel exhaust contain three main components. One component is the solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous fraction contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The SOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel. The third component of the particulate matter is the so-called sulfate fraction, which is formed from small quantities of sulfur components present in the diesel fuel.
Catalyst compositions and substrates on which the compositions are disposed are typically provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For instance, oxidation catalysts that contain platinum group metals, base metals and combinations thereof facilitate the treatment of diesel engine exhaust by promoting the conversion of both unburned hydrocarbons (HC) and carbon monoxide (CO) gaseous pollutants, and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been disposed on various substrates (e.g. honeycomb flow through monolith substrates), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. Certain oxidation catalysts also promote the oxidation of NO to NO2.
In addition to the use of oxidation catalysts, diesel particulate filters are used to achieve high particulate matter reduction in diesel emissions treatment systems. Known filter structures that remove particulate matter from diesel exhaust include honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust. Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite or silicon-carbide. Wall flow substrates are particularly useful to filter particulate matter from diesel engine exhaust gases. A common construction is a multi-passage honeycomb structure having the ends of alternate passages on the inlet and outlet sides of the honeycomb structure plugged.
This construction results in a checkerboard-type pattern on either end. Passages plugged on the inlet axial end are open on the outlet axial end. This permits the exhaust gas with the entrained particulate matter to enter the open inlet passages, flow through the porous internal walls and exit through the channels having open outlet axial ends. The particulate matter is thereby filtered on to the internal walls of the substrate. The gas pressure forces the exhaust gas through the porous structural walls into the channels closed at the upstream axial end and open at the downstream axial end. The accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure.
Catalyst compositions deposited along the internal walls of the wall flow substrate assist in the regeneration of the filter substrates by promoting the combustion of the accumulated particulate matter. The combustion of the accumulated particulate matter restores acceptable back pressures within the exhaust system. These processes may be either passive or active regeneration processes. Both processes utilize an oxidant such as O2 or NO2 to combust the particulate matter.
Passive regeneration processes combust the particulate matter at temperatures within the normal operating range of the diesel exhaust system. Preferably, the oxidant used in the regeneration process is NO2 since the soot fraction combusts at much lower temperatures than those needed when O2 serves as the oxidant. While O2 is readily available from the atmosphere, NO2 can be actively generated though the use of upstream oxidation catalysts that oxidizes NO in the exhaust stream.
In spite of the presence of the catalyst compositions and provisions for using NO2 as the oxidant, active regeneration processes are generally needed to clear out the accumulated particulate matter, and restore acceptable back pressures within the filter. The soot fraction of the particulate matter generally requires temperatures in excess of 500° C. to burn under oxygen rich (lean) conditions, which are higher temperatures than those typically present in diesel exhaust. Active regeneration processes are normally initiated by altering the engine management to raise temperatures in front of the filter up to 570-630° C.
During the passive regeneration on current state of the art catalyzed soot filter, the NO2 consumed during the oxidation of soot can be produced again by the catalyst assisted oxidation of NO along the channel of the catalyzed soot filter. In order to provide sufficient NO2 to oxidize soot and avoid frequent active soot regenerations, Pt-rich washcoats have been applied on the soot filter material. However, such Pt-rich washcoats raise concerns due to risk of producing a high amount of NO2 which would exit the catalyzed soot filter without being used for the oxidation of soot. The NO2 exiting the catalyzed soot filter can be emitted in the atmosphere only if its concentration fulfills the requirements of the air regulation limits, otherwise its concentration has to be reduced or the NO2 converted by means of further downstream catalysts such as NOx traps and/or catalysts able to selectively reduce NOx in presence of urea, ammonia or hydrocarbons. The need of abating the NO2 emission is not only limited to the normal operation of a diesel engine but also during the so called active regenerations. In fact, during the high temperature oxidation of soot by oxygen, NO2 produced on the Pt-rich washcoat can not be fully consumed by reaction with soot.
EP-A-1 541 219 discloses a catalyzed soot filter which would simultaneously remove soot and NOx by combination of NOx storage catalysts with the soot filter. This solution is however disadvantageous in that it additionally requires the use of another precious metal, e.g. Ag and/or base metal oxides, for the storage and conversion and/or release of NOx or to limit the NO2 conversion, which not only add complexity and increase costs but also lead to a more sulfur sensitive system. In fact, the sulfur present in the commercially available diesel fuel could poison the activity of Ag, therefore forcing the system to be more frequently regenerated and thus to have a higher fuel penalty.
EP 1 837 076 A1 and JSAE 20077233 disclose a catalyzed soot filter formulation which suppresses the NO2 formation during active filter regeneration as well as during normal diesel engine operation. Such suppression is achieved by the use of mixed base metal oxides e.g. Cu, La—Cu, Co and Fe oxides comprised in a PGM containing washcoat. Also in this case, the disadvantages come from the use of such base metal oxides which render the system more sulfur sensitive or less able to fully oxidize CO and HC.
Alternative methods to remove soot and NOx during the engine operation rely on the use of the so called SCR (selective catalytic reduction) catalysts, which can be separated from the soot filter or integrated into it. In both cases, these methods do not provide an optimal solution which could be widely applied. In fact, while separating the SCR catalysts from the catalyzed soot filters could be advantageous to specifically address the abatements of discrete components in the exhaust system, the increased cost, need of reductant and increased volume of such a system limits its applicability. On the other side, when the SCR catalyst is implemented into a catalyzed soot filter, although the system volume is reduced, there is an increased risk of having unacceptably high back pressure in the exhaust line as well as still the need of a reductant to be injected into the system.
Therefore, it would be desirable to provide an improved catalyzed soot filter which ensures oxidation of soot via NO2 during normal diesel engine operation and also suppresses the NO2 formation reaction during active regeneration. Moreover, it would be desirable to provide a catalyzed soot filter which ensures that the concentration of unconverted NO2 exiting the catalyzed soot filter is as low as possible in order to fulfill the air regulation limits, preferably without the need of an additional NOx reduction system. Thus, the catalyzed soot filter should provide an economically more favorable NO2 abatement. Additionally, it would be desirable to provide a catalyzed soot filter which, apart from controlling the NO2 formation reaction, continually supports the oxidation and abatement of CO and unburned HC—and thus allows for a minimum breakthrough of HC and CO—as well as maintains its soot filtration capabilities. Finally, it would be desirable to provide a catalyzed soot filter which, due to the rarity and consequently costs of precious metal components usually used for the preparation of catalyzed soot filters, contains a reduced amount of platinum in the catalyst composition allowing for reduced costs for the catalyzed soot filter without reducing the filter efficiency.